Billy Twelvetrees has become the fourth replacement called up by the British and Lions inside six days as injury concerns continue to disrupt the touring team's preparations for next week's first Test against Australia. The Gloucester centre has been summoned as cover amid continuing uncertainty over the current fitness of Jonathan Sexton, Owen Farrell and Manu Tuilagi.

The 24-year-old Twelvetrees can play at centre or fly-half and was within an ace of being selected in the original party. After a fine game for England in their win over Argentina last weekend, he is now due to arrive in Sydney on Friday, barely 24 hours before the Lions face the NSW Waratahs in their penultimate match prior to the Test series.

The starting line-up for the Waratahs fixture has yet to be confirmed but Gatland's squad clearly has lingering injury issues with the first Test 10 days away. "The nature of a Lions tour means there will inevitably be knocks which take time to recover from," said Gatland.

"The whole squad have worked hard but a lot of the backs have doubled up over the last two games, either starting or from the bench. Billy played well in the first Test for England and he provides that versatility to cover a variety of positions."

From the moment Gatland named only two specialist fly-halves and two inside centres there was always a possibility the Lions's resources would be stretched at some stage. Neither James Hook nor Jonny Wilkinson is touring with their respective countries, leaving Twelvetrees and Ireland's Ian Madigan as the leading candidates. The England player has won just five caps since making a try-scoring debut against Scotland in February but his passing and kicking games are outstanding and he also possesses the physical attributes to match.

He will now join the two loose-head props Alex Corbisiero and Ryan Grant and the Ireland wing Simon Zebo in the Lions party, as the tour medics continue to treat Farrell (dead leg), Sexton (hamstring) and Manu Tuilagi (shoulder). Twelvetrees, who also scored a try in England's 32-3 victory in Salta, will fly out of Argentina with the blessing of his national coach Stuart Lancaster. "All the England players and management are delighted for Billy," said Lancaster. "He's played well for England this year, underlined that form with his display against Argentina at the weekend and goes with our full support and blessing." Bath's Kyle Eastmond is now poised for his first start at inside centre for England against the Pumas in Buenos Aires this weekend.